Method and apparatus for dissolved air flotation and related waste water treatments

ABSTRACT

A dissolved air flotation (DAF) system and method for gas-liquid contacting operations. A mixture of untreated liquid and liquid saturated with dissolved air is passed through a series of upward deflecting screens placed across the flowpath in a rectangular tank. Microbubbles of air released from the mixture produce a buoyant force which carries contacted particles to the surface. Floated particles released from the microbubbles form a sludge which is supported above the surface of the mixture by a layer of air derived from the microbubbles. A top layer of the sludge is skimmed off according to a predetermined residence time of sludge at the surface. Liquid containing dissolved air is also introduced downstream of each deflecting screen to enhance flotation and clarification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to treatment of waste liquids; and moreparticularly to a new and improved method and apparatus for treatingwaste water and other liquids utilizing dissolved air flotation andrelated technologies.

2. Description of the Prior Art

Conventional methods of treating waste water and similar liquids isusually carried out in several stages. Easily settled solids areseparated from a liquid medium by sedimentation, and suspended solidsand emulsified matter are removed by unassisted flotation or dissolvedair flotation (DAF). Remaining dissolved matters may be convertedchemically to precipitates, or biologically to microorganisms, forseparation from the liquid medium. Sometimes dissolved matters areremoved by mass transfer operations, such as adsorption, desorption,stripping, extraction, crystallization, membrane separation, etc. Thesolids separated by these treatments is usually dewatered beforeincineration or by other disposal means.

The equipment employed in each stage of treatment requires considerablecapital investment and plant space, especially if the treatment involvesaerobic or anerobic biotreatments of dissolved organics or oxidativedigestion of microorganisms. Such treatments are not possible wherethere is limited capital and space.

Most of these systems are also unacceptable economically andenvironmentally. They are relatively inefficient because of their lowconversion rates, due mainly to turbulence and back-mixing in the liquidflowpath, especially at high flow rates. The pervasiveness of theturbulence and back-mixing is discussed in more detail in U.S. Pat. No.5,382,358 to George C. Yeh entitled "Apparatus for Dissolved AirFlotation and Similar Gas-Liquid Contacting Operations."

Conventional DAF systems typically place baffles inside a flotation tankin front of the incoming liquid medium to direct floc-bubbleagglomerates formed therein toward the surface. In an uprightcylindrical tank configuration, the baffle is generally placedconcentrically and upright in the tank. In a rectangular tankconfiguration, such as disclosed in U.S. Pat. No. 3,175,687 to W. H.Jones, the baffle is flat and placed generally upright. The turbulence,recirculation and back-mixing produced by the baffles more than offsetsthe beneficial effect of directing the agglomerates to the surface,especially at high flow rates or sudden surges in flow. Consequently,the liquid loading capacity is limited, and clarification efficiencydegraded because the incoming liquid continues to flow freely underturbulent conditions after passing the baffles. While such baffles inboth cylindrical and rectangular flotation tanks help to directfloc-bubble agglomerates toward the surface of the liquid medium, theirtotal effect is detrimental to DAF systems.

The effects of turbulence and back-mixing can be reduced by increasingthe length of the liquid flowpath over which flotation occurs, such asby a long rectangular tank with liquid inlet and outlet ports at eitherend, but space, especially if indoors, is insufficient to accommodatesuch a long tank. Another tank designed for smaller areas utilizes ashort rectangular tank divided lengthwise into a connecting series oflabyrinthine-like compartments through which the liquid flows in awinding path. However, the turbulence created at the junctions of thesecompartments, and the restrictions in the number of compartmentspossible, limit the beneficial effects.

U.S. Pat. No. 5,382,358 supra discloses a unique design in which anupright cylindrical flotation tank includes at least one partitioncurved forming a long involute or evolute flow channel. A mixture ofuntreated liquid and recycled liquid saturated with dissolved air underpressure is continuously introduced at a controlled rate at one end ofthe channel and flows through the channel under near plug-flowconditions. Micro-bubbles of air released from the mixture rise throughthe liquid mixture providing a buoyant force which carries any contactedparticles of suspended matter to the surface. A sludge is formed and thetop layer of which is continuously skimmed off at a rate regulated tomaintain a predetermined residence time and solid content. Centrifugaland gravitational forces in the flowpath cause large or heavy solids toseparate and settle in the bottom of the tank where they may be drawnoff. Commercial units constructed according to the patent have achievedunusually high clarification efficiencies, viz. at or near 100% byweight, and solid contents in the sludge between 50% and 85% by weightdepending on the sludge residence time on the surface.

The involute or evolute flotation tank configuration is also applicableto gas-liquid contacting processes, mass transfer processes betweengases and liquids, chemical and biological gas-liquid reactions, and fortwo or more of these processes carried out simultaneously. No other DAFsystems are known to be capable of carrying out multiple processessimultaneously.

While the involute or evolute configuration tank has proven asignificant improvement in the state of the art, there remain new DAFfacilities where rectangular flotation tanks are more suitable becauseof lower initial costs, anticipated loadings, nature of the liquidmedium, etc. There are also many existing DAF facilities havingrectangular flotation tanks in need of upgrading for more efficientoperations.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a noveland improved DAF system which utilizes a rectangular flotation tankconfiguration for treating waste water and other liquids, which yieldhigh clarification efficiencies and high solid contents in the floatedsludge.

Another object is to provide an improved DAF method and apparatusutilizing a rectangular tank configuration for gas-liquid contactingprocesses and chemical and/or biological gas-liquid reactions.

A still further object of the invention is to provide an improvedrectangular flotation tank for a DAF liquid treatment process capable ofsimultaneously clarifying treated liquid, thickening floated sludge andreacting the liquid with a gas.

Still another object of the invention is to provide an improved DAFmethod utilizing a rectangular flotation tank configuration forachieving high clarification efficiencies at very high liquid loadingrates in a short flowpath.

Briefly, these and other objects are achieved in a DAF system comprisingan upright rectangular flotation tank in which a mixture of untreatedliquid and liquid saturated with dissolved air under pressure isintroduced at one end of the tank and treated liquid is discharged atthe other end of the tank near the bottom. A series of deflector screensplaced across the flowpath at spaced intervals direct the liquid mixtureslightly upward to produce vertically uniform near-laminar flow withlittle or no turbulence. The distance between adjacent screens isusually determined by the height of the liquid level and the averagelinear velocity of the liquid flowing through the tank. The flow isdivided into several parallel channels to insure more uniformnear-laminar flow. Microbubbles of air released from the liquid mixtureflowing through the tank produce a buoyant force sufficient to carry anycontacted particles of suspended matter to the surface.

The near-laminar flow of the liquid produces a sliding action betweenparticles contained in adjacent layers which aids in hydraulicflocculation of the particles. Under steady state conditions a steepclarification gradient is thereby established vertically andhorizontally along the flowpath with least concentration of particles atthe discharge end of the tank near the bottom. The deflector screenshelp prevent non-ideal flows, such as circulation, short-circuiting,by-passing, back-mixing, etc. which commonly occur in the prior artrectangular tank DAF systems. Such non-ideal flows must be absent toproduce stable particle-microbubble attachment and plug-like flow in thetank for high clarification efficiencies and/or conversion.

The liquid height and residence time of the floated matter is regulatedto form a bed of the sludge supported by a blanket of the rising bubblesabove the liquid surface. As the sludge bed thickens, the region nearthe top becomes drier and is removed by a slow-moving skimmer at a rateequal to the flotation rate in order to maintain the sludge bedthickness.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the objects and principles of theinvention, reference will be made to the following detailed descriptiontaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic block diagram of a preferred embodiment of a DAFsystem according to the invention utilizing a multichannel rectangularflotation tank;

FIG. 2 represents an elevation view in longitudinal cross section of theflotation tank of FIG. 1;

FIG. 3 represents a plan view in longitudinal cross section of theflotation tank taken along the line 3--3 of FIG. 2;

FIG. 4 is an elevation view of typical near-laminar flowpaths producedin the flotation tank of FIG. 1; and

FIG. 5 is a velocity profile through a vertical section of a flowpath.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like referenced characters andnumerals designate like or corresponding parts throughout the severalviews, there is shown in FIG. 1 a DAF system, indicated generally by thenumeral 10, with an open-top flotation tank 12 for treating a liquidL_(m) according to the invention. As shown in more detail in FIGS. 2 and3, the configuration of tank 12 is generally rectangular with uprightside walls 12a and 12b, end walls 12c and 12d, and a bottom hopper 12ewhich slopes downward to a drain 11 for collecting and dischargingsediment. Parallel flow channels are formed by vertical partitions 13extending substantially the full length of tank 12 without touchingbottom 12e or extending above the height of the level of a mixed liquidL_(m). In tank 12 the partitions provide narrow channels for reducinglateral migration of the flowpath as well as maintaining more uniformnear-laminar flow. Partitions 13, of course, may be omitted where thewidth of the tank is narrow and/or flow rates are low.

Influent or untreated liquid L₁ passes through an orifice 14 and entersa mixing chamber 16 through an inlet port 18 at the bottom of tank 12where it is mixed with a treated and recirculated liquid L₂ containingdissolved air which enters chamber 16 through an inlet port 20 laterallyadjacent to port 18. Recirculated liquid L₂ is derived from a portion ofclarified mixed liquid L₃ discharged from an outlet port 22 at thebottom of tank 12 at rate controlled by a valve 23. Compressed air froma supply 26 is contacted and dissolved in liquid L₃ as it is deliveredunder high pressure through a pump 30 and a dissolution and separationtank 31 to inlet port 20. For anaerobic treatments, nitrogen may besupplied to the tank instead of air. A pressure relief valve 32 at thetop of tank 31 allows any undissolved gas to escape.

An alternative or back-up system for dissolving air in recirculatedliquid L₂ includes an air injector 34 through which the liquid L₃ iscirculated by pump 30. As shown, either arrangement may be used as aback-up for the other. For example injector 34 operates by closingshut-off valve 36 at the outlet of compressed air supply 26 and openingshut-off valves 37 at the inlet and outlet of air injector 34. Byregulating the air injection rate, the formation of undissolved air canbe prevented.

The mixture of liquids L₁ and L₂ forms liquid L_(m) which flows intotank 12 under turbulent conditions through a plurality of holes 16a inchamber 16 with a large portion forming floc-bubble agglomerates inliquid L_(m) which quickly rise to the surface. The remainder flowsthrough a series of deflector screens or baffles 38 placed across thechannels at spaced intervals along the flowpath.

Each baffle 38 includes a plurality of vertically spaced vanes 39. Eachvane 39 comprises segments in respective channels fixed to each other ona common axis extending horizontally through openings in partition 13and rotatably supported at its ends in vibration insulators 41 fixed totank sidewalls 12a and 12b. The profile of each vane 39 is streamlinedto minimize drag and both the leading and trailing edges are preferablyacute to minimize boundary layer separation and turbulence.

The distance between adjacent screens is determined according to theaverage linear velocity of liquid L_(m) and the average rising velocityof escaping air bubbles. In other words, the distance is set so that theaverage time for the liquid L_(m) to travel between the adjacent screens38 is approximately equal to the average time for all the air bubblesbetween the baffles to rise to the top of liquid L_(m). The spacingbetween individual vanes 39 is determined according to the liquidloading rate and may vary approximately between a few centimeters forsmall flow rates and 30 centimeters for very large flow rates.

The vertical angle of each deflector vane 39 in each screen 38 isadjustable by gang-operated levers 40 according the height and flowrateof liquid L_(m). The angle may vary approximately between a few degreesfor very shallow tanks and 60 degrees for very deep tanks. Levers 40 mayalso be adjusted automatically to increase and decrease directly withchanges in the flowrate of liquid L_(m).

As shown in FIG. 4, mixed liquid L_(m) is unified and deflected slightlyupward as it flows through the screens 38 thereby quickly reducing theextensity and intensity of turbulence and shockwaves near the mixingchamber 18 to produce more uniform and near-laminar flow. The mainfunctions of deflector baffles 38 are to resist shock loadings of liquidand dump liquid turbulences, to prevent back-mixing, to direct the flowtoward the surface, to unify the axial flow, and prevent variousnon-ideal flows and bring the flow closer to vertically uniform laminarflow or so-called plug-like flow. Therefore, the more turbulent theincoming liquid is, the more sets of the deflector baffles are needed toproduce the above effect.

Strictly speaking, the uniformity of the laminar flows produced shouldincrease as the number of deflector vanes per set increases also. Sincethe mixing chamber is designed and placed in such a way as to producehorizontally even (uniform) flows, the extensity and intensity of theturbulence of the incoming liquid may be relatively even in thehorizontal direction. But, in the vertical direction, the turbulence ofthe incoming liquid is both intensive and extensive. The deflectorbaffles 38 reduce the turbulence. Important design parameters are theinitial liquid turbulence, Reynolds number, design of the deflectorbaffles, the number of baffles in the flow path, and the number ofpartitions 13 needed for reducing the Reynolds number, Re. Re is reducedwhen the liquid flows through the space between vanes 39. Over ahorizontal surface, as illustrated in FIG. 5, the net result of thesefunctions is plug-like flow conditions having a velocity profile p-1 ascompared to the velocity profiles for ideal plug flow p, laminar flow 1and turbulent flow t. Essentially, plug-like flow profile p-1 comprisesa plurality of small laminar flow profile between each of adjacentdeflector vanes 39.

Dissolved air in liquid L₂ is also introduced evenly into mixed liquidL_(m) downstream of each deflector baffle 38 near the bottom of tank 12through spaced apart nozzles in a distribution conduit 43 extendingacross the flowpath. In a small capacity tank, only one or two deflectorbaffles 38 and distribution conduits 43 may be needed to preventback-mixing, but more baffles and conduits may be required as the tanklength and liquid loading rate increase.

The discharge end of tank 12 includes a baffle 42 extending betweensides 12a and 12b with an opening 42a across the bottom for most of theclarified liquid L₃ to discharge through an outlet port 44 to anoverflow box 48. An adjustable weir 46 in box 48 maintains the level ofliquid L_(m) at a predetermined height. Other means for controlling theliquid level are contemplated such as a liquid level sensor regulatingthe rate of untreated liquid L₁ at input port 18.

Many microbubbles of air, released from the distribution conduits 43evenly supplied throughout tank 12, will attach and completely surroundparticles in suspension to form segregated floc-bubble agglomerates. Theagglomerates float to the surface of liquid L_(m) to form a bed ofsludge S. The agglomerates are not easily wetted because they aresurrounded by the microbubbles, and will stay dry as they float to thesurface and attach to the bottom side of the sludge bed. Theseconditions assure a high clarification efficiency and high content ofsludge solids. As the agglomerates contact the bottom side of the sludgebed, the air bubbles are released from the agglomerates and accumulateas a blanket of air A separating the bottom of sludge S from the surfaceof liquid L_(m). The air blanket helps to prevent sludge S from breakingup and remixing with the liquid. The concentration of agglomeratesdirectly under air blanket A is relatively high due to the flotationaction of the rising air bubbles. By maintaining a thick bed of sludge Sand separating it from the liquid surface by air blanket A, the toplayer of sludge S can be easily skimmed off without disturbing the wholesludge bed.

The sludge S is slowly compressed under its own weight against theopposing buoyancy force applied by the blanket of air A. The sludge Sbecomes drier near the top as water drips slowly downward undercompression or evaporates.

While the sludge bed stays afloat on the air blanket A, the top layer isskimmed off by a continuous chain of skimmer blades 50 driven bymotor-drive sprockets 52 at either end of tank 12 preferably in theopposite direction to the flow of liquid L_(m). The speed of sprockets52 is regulated by a motor speed controller 53. The top layer of sludgeS travels over a weir plate 54 and discharges into a trough or hopper 56across the width of end wall 12c. Blades 50 are preferably constructedof Teflon, or rubber supported between stainless steel plates, the depthof penetration into sludge S being adjustable such as by changing theelevation of sprockets 52 by a rack and pinion mechanism 55 or theheight of the liquid level. The removal rate sludge S can be controlledby adjusting the liquid height, blade speed and penetration depth, tomatch the flotation rate. Thus, the residence time of sludge S in thetank is controlled so that the solids content of the removed sludge isalso controlled. Upper sections of end walls 12c and 12d may be slopedinwardly over sprockets 52 for an operator's protection. Float sludgesproduced by this DAF system have solid contents between 50% and 85%,compared to less than 10% in DAF systems of the prior art.

The DAF system according to the invention is also applicable tooperations involving gas-liquid contacting or chemical reactions. Insuch a system the top of flotation tank 12 is covered in order torecover and discharge, or recycle, any vapors or gases given off.Typical industrial applications include, but not limited to,disinfecting water with a biocide, such as chlorine, chlorine dioxide,or a non-oxidizing biocide; oxidating waste water with ozone, oxygen,oxygen-rich air; striping or desorption of dissolved gases and dissolvedvolatile organic matter from water or other liquids; scrubbing of aircontaining solid particles and/or water-soluble gases such as ammonia,hydrogen sulfide, carbon oxide, carbon monoxide, etc.; and separation ofgas mixtures by absorption with water or other liquid.

It is important to point out that the rate of mass transfer between agas phase and a liquid phase in gas-liquid contacting operations and ingas-liquid reactions can be increased many times if dissolved gas isused. This holds true regardless of the direction of mass transfer, i.e.from gas-to-liquid or from liquid-to-gas. In the case of chemical orbiological gas-liquid reactions, the activities of dissolved gasreactants are usually the driving force in the reactions; therefore, theuse of dissolved gas reactants can increase the reaction rates greatly.This fact is well-known for gas-liquid chemical reactions, but not forbiological reactions. Experiments have shown that the use of dissolvedair in biotreatments of waste water can increase the rate of thebio-reaction hundredfolds. Accordingly, it is extremely advantageous touse a DAF system to carry out gas-liquid mass transfers or gas-liquidreaction, both chemical and biological. To increase the treatmentcapacity of the flotation tank as a bioreactor, deflector baffles 38 mayalso serve as a convenient attachment for the growth of microorganismsneeded for biotreatments.

When applied to an aerobic or anaerobic biotreatment process, part ofthe suspended solids, consisting of mostly microorganisms, accumulatedunder the air blanket A, is continuously recycled to inlet port 18through a recirculation pipe 58, the rate of recirculation beingcontrolled by valve 60. The kinetic energy for recirculation isgenerated by untreated liquid L₁ passing through an orifice 14.Alternatively, a recycling pump (not shown) may provide therecirculation through pipe 58. The recycling rate determines the extentof bioreaction and rate at which final inactive sludge and gas productare produced. Of course, a part of sludge S discharged at trough 56 canbe recycled by means not shown directly into tank 12 or through theinlet port 18. The gases formed are discharged from tank 12 in the sameway as for a gas-liquid contacting process.

The apparatus provided by the invention can be located and operatedindoor, or outdoor with a protective cover. By comparison with aconventional rectangular DAF apparatus, its length is shorter for largeliquid loadings. Therefore, it occupies less space and can be installedon a site having very limited area.

The near-laminar flow of liquid through tank 12 produces a hydraulicflocculation effect on the particles, thus requiring less coagulatingagents and flocculating agents in pretreatment of raw waste liquids. Theplug-like flow under uniform near-laminar flow conditions enables fasterseparation with higher efficiencies.

Most of the existing rectangular DAF system can be readily retrofittedat low cost with deflector baffles 38 and the dissolved air distributors43 and achieve the desired effects.

The apparatus as disclosed can also be used for simultaneously carryingout various processes and operations with high rates of efficiency, suchas clarification, thickening and dewatering of sludge, gas-liquidcontacting, mass transfers and chemical or biological gas-liquidreactions.

It will be understood that various changes in the details andarrangement of parts, which have been herein described and illustratedin order to explain the principles of the invention, can be made bythose skilled in the art within the principles and scope of theinvention as defined in the applied claim.

I claim:
 1. In an improved dissolved air flotation system for removingsuspended matter contained in a first liquid, the system including arectangular tank forming a continuous flowpath between opposed ends, aninlet port at one of the ends for receiving a liquid mixture of thefirst liquid and a second liquid containing dissolved air, a firstoutlet port at the other of the ends for discharging any of the liquidmixture separated from the suspended matter, and a second outlet port ateither of the ends for discharging the suspended matter in the liquidmixture floated to the surface by microbubbles of air released along theflowpath, the improvement comprising:baffle means including a pluralityof vanes defining inclined surfaces disposed within the flowpath forredirecting the liquid mixture upwardly and forwardly in a continuousstream of plug-like flow without back-mixing, each vane beingsubstantially shorter than the length of the tank.
 2. The improvementaccording to claim 1 wherein:said baffle means includes a vertical arrayof streamline vanes spaced apart on horizontal axes across the flowpath.3. The improvement according to claim 2 further comprising:adjustingmeans for jointly regulating the angular position of said vanes aboutthe axes in the flowpath according to the liquid level and flow rate ofthe mixture.
 4. The improvement according to claim 2 wherein:each ofsaid vanes is relatively thin in cross section and has acute leading andtrailing edges.
 5. The improvement according to claim 1 furthercomprising:distributor means extending across the flowpath near thebottom of the tank and downstream of the said baffle means forintroducing in the liquid mixture a supplemental amount of the secondliquid.
 6. The improvement according to claim 5 wherein:said distributormeans includes a conduit with a plurality of apertures spaced along thelength thereof.
 7. The improvement according to claim 1 furthercomprising:regulating means for adjusting the liquid level of themixture and the residence time of floated matter for spatiallysupporting the floated matter above the surface of the liquid mixture inthe tank by a blanket of the released air.
 8. The improvement accordingto claim 7 wherein:said regulating means includes a weir connected tosaid first outlet port for controlling the liquid output from said tank,and an extractor controller for regulating the amount of floated matterremoved from the tank.
 9. The improvement according to claim 8wherein:said liquid level controller includes an adjustable weirconnected to said outlet port for regulating the liquid outputtherefrom.
 10. The improvement according to claim 8 wherein:saidextractor controller includes a plate connected to said second outletport for preventing a bottom layer of the floated matter fromdischarging, and motor-driven chain of blades for skimming off a toplayer of the floated matter at one of the ends of the tank, theelevation of said chain being adjustable.
 11. The improvement accordingto claim 10 wherein:said extractor controller further includes a bladespeed regulator for maintaining a preselected rate of removal a toplayer of the floated matter.
 12. The improvement according to claim 1wherein:said baffle means includes a plurality of vertical arrays ofstreamline vanes positioned at spaced intervals along the flowpath. 13.The improvement according to claim 12 wherein:said vanes in each of saidarrays are vertically spaced apart on horizontal axes thereof.
 14. Theimprovement according to claim 13 further comprising:adjusting means forjointly regulating the vertical angular position of said vanes about theaxes in the flowpath according to the liquid level and flow rate of themixture.
 15. The improvement according to claim 13 wherein:each of saidvanes is relatively thin in cross section and has acute leading andtrailing edges.
 16. An improved dissolved air flotation tank forremoving suspended matter contained in a first liquid, comprising:arectangular vessel forming a lengthwise flowpath between opposed ends;an inlet port at one of the ends for receiving a liquid mixture of thefirst liquid and a second liquid containing dissolved air; a firstoutlet port at the other of the ends for discharging any of the liquidmixture separated from the suspended matter; a second outlet port ateither of the ends of the tank for discharging the suspended matter inthe liquid mixture floated to the surface by microbubbles of airreleased along the flowpath; at least one vertical array of streamlinevanes disposed on horizontal axes across the flowpath between oppositesides of the tank, each of said vanes having an inclined surface in theflowpath substantially shorter than the length of said vessel forredirecting the mixture upwardly and forwardly in a continuous plug-likeflow without back-mixing.
 17. A flotation tank according to claim 16wherein:said vanes are angularly positioned about the axes.
 18. Aflotation tank according to claim 17 further comprising:adjusting meansfor jointly regulating the angular position of said vanes with respectto the flowpath.
 19. A flotation tank according to claim 17 wherein:eachof said vanes is relatively thin in cross section and has acute leadingand trailing edges.
 20. A flotation tank according to claim 16 furthercomprising:distributor means extending across the flowpath near thebottom of the tank and downstream of each of the said at least onevertical array of vanes for introducing supplemental amount of thesecond liquid into the liquid mixture.
 21. A flotation tank according toclaim 20 wherein:said distributor means includes a conduit with aplurality of apertures spaced along the length thereof.
 22. A flotationtank according to claim 16 further comprising:regulating means foradjusting the liquid level of the mixture and residence time of thefloated matter for spatially supporting the floated matter above thesurface of the liquid mixture in the tank by a blanket of the releasedair.
 23. A flotation tank according to claim 22 wherein:said regulatingmeans includes a weir connected to said first outlet port forcontrolling the liquid output from said tank, and an extractor forregulating the amount of floated matter removed from the tank.
 24. Aflotation tank according to claim 23 wherein:said liquid levelcontroller includes an adjustable weir connected to said outlet port forregulating the liquid output therefrom.
 25. A flotation tank accordingto claim 23 wherein:said extractor includes a trough disposed across theone end of the tank above the liquid level, and a motor-driven chain ofblades for discharging a top portion of the floated matter into saidtrough, the elevation of said chain being adjustable.
 26. A flotationtank according to claim 25 wherein:said extractor further includes ablade speed regulator for maintaining a preselected rate of removal ofthe floated matter.
 27. A flotation tank according to claim 26 furthercomprising:plate means mounted in said trough for limiting the floatedmatter discharged to the top portion thereof.
 28. A dissolved airflotation system for separating suspended matter from a liquid,comprising, in combination:a rectangular tank forming a flowpath betweenopposed ends with an inlet means at one of the ends; a mixing chamber atthe one end communicating with said inlet means for introducing amixture of untreated liquid containing suspended matter with treatedliquid containing dissolved air into the flowpath along the length ofsaid tank; baffle means including a plurality of vanes defining inclinedsurfaces disposed within the flowpath for redirecting the liquid mixtureupwardly and forwardly in a continuous stream of plug-like flow withoutback-mixing, each vane being substantially shorter than the length ofthe tank; skimming means disposed in an upper section of the tank forremoving a top layer of the suspended matter carried to the surface bymicrobubbles of air released from the mixture and adhered to thesuspended matter; an outlet port of the other of the ends fordischarging treated liquid separated from the matter floated to thesurface along the flowpath; means for dissolving air in a portion of thetreated liquid; and recycling means connected to said inlet port meansfor recycling said portion to said mixing chamber.